The present invention relates generally to marine seismic surveying. More particularly, in one or more embodiments, this invention relates to marine seismic vibrators for use in seismic surveying and associated methods of use.
Techniques for marine surveying include marine seismic surveying, in which geophysical data may be collected from below the Earth's surface. Seismic surveying has applications in mineral and energy exploration and production to help identify locations of hydrocarbon-bearing formations. Seismic surveying typically may include towing a seismic source below or near the surface of a body of water. One more “streamers” may also be towed through the water by the same or a different vessel. The streamers are typically cables that include a plurality of sensors disposed thereon at spaced apart locations along the length of each cable. Some seismic surveys locate sensors on ocean bottom cables or nodes in addition to, or instead of, streamers. The sensors may be configured to generate a signal that is related to a parameter being measured by the sensor. At selected times, the seismic source may be actuated to generate, for example, seismic energy that travels downwardly through the water and into the subsurface rock. Seismic energy that interacts with interfaces, generally at the boundaries between layers of rock formations, may be returned toward the surface and detected by the sensors on the streamers. The detected energy may be used to infer certain properties of the subsurface rock, such as structure, mineral composition and fluid content, thereby providing information useful in the recovery of hydrocarbons.
Most of the seismic sources employed today in marine seismic surveying are of the impulsive type, in which efforts are made to generate as much energy as possible during as short a time span as possible. The most commonly used of these impulsive-type sources are air guns that typically utilize compressed air to generate a sound wave. Other examples of impulsive-type sources include explosives and weight-drop impulse sources. Another type of seismic source that can be used in seismic surveying includes vibrator sources, including hydraulically powered sources, electro-mechanical vibrators, electrical marine seismic vibrators, and sources employing piezoelectric or magnetostrictive material. Vibrator sources typically generate vibrations through a range of frequencies in a pattern known as a “sweep” or “chirp.”
It is well known that as sound waves travel through water and through subsurface geological structures, higher frequency sound waves may be attenuated more rapidly than lower frequency sound waves, and consequently, lower frequency sound waves can be transmitted over longer distances through water and geological structures than higher frequency sound waves. Accordingly, there has been a need for powerful low frequency marine sound sources operating in the frequency band of 1-100 Hz. However, both impulsive-type and vibrator sources that have been used may generate little or no energy below 20 Hz. In addition, low frequency sources typically may have poor efficiency, especially if a good impedance match cannot be obtained.
Thus, there is a need for improved seismic sources for use in marine surveying.